[sdiy] sync conditioning/simple triangle oscillator without linear voltage control

David Moylan dave at expeditionelectronics.com
Fri Feb 22 19:08:45 CET 2019

This issue has been tread a lot in the realm of guitar "octavers", 
mainly octave down effects, and guitar synthesizers.  The most popular 
circuit for guitar octavers seems to be the peak detector circuits 
rather than zero crossing detection.   Google the schematic for the Boss 
OC-2 for that type.  Some guitar synthesizers like Roland's GR-300 used 
zero crossing detection, but had separate processing per string which 
makes the job easier since you only need good tracking for ~2 octaves.  
They also used filters that would switch center frequency based on the 
detected pitch.

The OC-2 method is certainly more economical circuitry-wise.  I've been 
curious lately about the history of that variable threshold detector.  
Earliest use I could find seems to be the ElectroHarmonix Octave 
Multiplexer from the seventies, but couldn't find any patents related to 
it.  Anyone know of earlier instances of the same or similar circuits?

úlfur, beautiful looking instrument, BTW.


On 2/22/19 8:56 AM, ulfur hansson wrote:
> hello richie,
> I already have constructed an instrument that works quite well, using 
> high powered class D amplifiers and custom wound electromagnets to 
> induce feedback in an acoustic set of strings. here is a brief 
> description of my harp, along with demo recordings; 
> https://ulfurhansson.com/SEGULHARPA-ELECTROMAGNETIC-HARP (kind of an 
> acoustic synthesizer/organ?)
> the internal circuitry has patchpoints for experimenting with 
> additional processing/signal conditioning. right now I am using 
> waveshapers to change timbre/harmonics as you press harder/lighter on 
> the capacitive touch sensor keyboard on the front panel.
> it works great, but i'd still like to explore other avenues for 
> varying timbre control. I feel my idea of a synced oscillator tethered 
> to each string is exciting as the sync artifacts will provide a much 
> richer harmonic response, that is if i can figure out how to implement 
> it with limited space and solve the challenge of signal conditioning 
> before SYNC input.
> it doesn't have to be perfect, and sync errors as the signal grows 
> stronger could actually be an interesting addition to the instrument 
> timbre. I would probably add a VCA to the oscillator output to control 
> the level of output mixed into the feedback signal depending on how 
> hard you press against the keyboard (aftertouch).
> thank you for your thoughts on this!
> all the best,
> -úlfur
> fös., 22. feb. 2019 kl. 11:43 skrifaði Richie Burnett 
> <rburnett at richieburnett.co.uk <mailto:rburnett at richieburnett.co.uk>>:
>     Synchronising an oscillator to a vibrating guitar string waveform
>     might be quite challenging. Different frequencies travel along
>     tensioned strings at different speeds leading to a dynamic
>     waveform with likely many zero-crossings per cycle. This is
>     particularly so just after plucking where string tension is
>     highest and high-frequency partials are loudest.
>     Attempts to extract pitch periods from harmonically complex
>     waveforms based on zero-crossings are usually of limited success
>     unless you can either heavily filter the input waveform to remove
>     harmonics, or use something like a PLL that will tend to
>     inherently reject harmonic energy in its reference signal if you
>     slug the loop filter enough.
>     Are you trying to make something like the e-bow?
>     -Richie,
>     Sent from my Xperia SP on O2
>     ---- ulfur hansson wrote ----
>     hello list,
>     I need to find a triangle oscillator design that meets two
>     requirements;
>     1 - it needs to have a sync input that can sync to a signal of
>     varying amplitude ( it is actually a single guitar string)
>     2 - it has to be tiny! I plan to design it using 0805
>     caps/resistors and SOT transistors etc...
>     they idea is to use this oscillator to reinforce a feedbacking
>     signal induced in the string using electromagnetic pickups and
>     actuators. I have already constructed an instrument that works
>     great, but I'd like to use this addtional circuitry to accentuate
>     different harmonics of the resulting tone by mixing the synced
>     oscillator in with the feedbacking string signal.
>     since the sync input signal is not a steady pure waveform,i will
>     probably need to do some signal conditioning - any ideas regarding
>     comparator designs for converting varying amplitude signal to
>     steady square wave would be greatly appreciated! perhaps a simple
>     diode compressor would be helpful too...
>     I hope this topic doesn't come across as too nebulous, but really
>     any ideas or helpful thoughts comments would be greatly appreciated!!
>     all the best,
>     -úlfur
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David Moylan
Expedition Electronics
sonic adventures!

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